1) Field of the Invention
The present invention relates generally to agricultural implements, and, more specifically, to lift and depth control systems for the frames of such implements.
2) Related Art
Agricultural implements such as field cultivators, chisel plows and seeders include frames having two or more sections hinged together and moveable between a narrow folded position and a wide field-working position. Lift wheel assemblies connected to the frame sections and controlled by hydraulic cylinders raise and lower the frame sections relative to the ground to move the implement between a raised transport position and a lowered field-working position and to control tillage or planting depth in the field-working position. The hydraulic cylinder circuit controlling the lift functions typically includes a plurality of cylinders connected to a selective control valve (SCV) on the tractor. The cylinders on some circuits are connected in series with each other so that the frame sections raise and lower in unison. However, many of these circuits rely on the cylinder pressure to maintain a selected frame height during operation, and cylinder leakage or leakage elsewhere in the circuit can cause unwanted variations in the operating depth of the implement. Depth control stops can be utilized to positively set the cylinder rod position for operation of the tools on the frame sections at the desired depth, but if the cylinders are plumbed in series for level lift, the entire system will be stopped when the first cylinder hits a stop and the remaining cylinders will ride on hydraulic fluid and thus be subject to leakage. If the cylinders are plumbed in parallel, movement of the sections between transport and field-working positions is uneven unless a mechanical link such as a rockshaft is connected between wheel assemblies or a complicated valve or flow control arrangement is added to the parallel circuit. The additional links or control arrangements add complexity and cost to the system. Other systems include a slotted or lost motion connection between the cylinder and wheel assembly which eliminates stall out when the stop is reached, but such systems typically do not allow cylinder pressure to hold the wheel assembly against the stop in the operating position and, with such systems, raising the wheels into the air is difficult or impossible.
Still other depth control systems utilize series cylinders which are fully extended when the frame sections are in the transport position and fully retracted when in the frame sections are in the transport position. These types of systems rely on cylinder anchor adjustments to make changes in the frame height in the operating position. The full retraction provides a positive stop in the field-working position so drift is eliminated and allows cylinder rephasing at the end of the stoke, but the anchor adjustment may also adversely affect transport height. In addition, most cylinder anchor adjustment mechanisms are relatively large and cumbersome.